The present invention relates generally to microfabrication techniques, and more particularly to methods for making a MEMS device.
Advancements in micromachining and other microfabrication techniques and processes have enabled the fabrication of a wide variety of MicroElectroMechanical Systems (MEMS) and devices. These devices include moving rotors, gears, switches, accelerometers, miniaturized sensors, actuator systems, and other such structures.
One popular approach for forming MEMS devices makes use of a modified wafer known as a Silicon-On-Insulator (SOI) wafer. An SOI wafer is essentially a silicon wafer having a sacrificial layer of silicon dioxide disposed thereon, and having a thin film of active single-crystalline silicon disposed on the sacrificial layer.
FIGS. 1-3 illustrate a conventional method for creating a MEMS structure on an SOI wafer. As shown in FIG. 1, an SOI wafer is provided which comprises a silicon wafer substrate 15 having a layer of thermally grown sacrificial oxide 13 such as silicon dioxide disposed thereon. A layer of active single crystal silicon 11 is disposed over the sacrificial oxide layer. The layer of active single-crystal silicon is then masked, patterned and selectively etched to define the basic structural features of the MEMS device as shown in FIG. 2. This structure will typically then be further processed to form various features, such as metal interconnects or isolation structures, on it. After the structure is completed, the sacrificial oxide layer is partially removed by selective chemical etching to release the structure. As shown in FIG. 3, the released MEMS structure 12 typically has a cantilevered portion 17 and an anchor portion 19.
The methods currently used for the release of SOI micromachined devices are limited by the maximum undercut distance achievable with commonly used etchants such as aqueous HF. The undercut distance is the distance over which the sacrificial oxide layer can be removed by an etchant, measured as the distance from the trench where the etchant was initially brought into contact with the sacrificial oxide layer. Since the release etch occurs after the device is completed, the undercut distance is limited by the slow etch rate of the sacrificial oxide by the etchant and by the selectivity of the etchant to other materials (such as silicon nitride and metals) that are used in the interconnects, isolation structures, and other features on the completed device. Due to these limitations in the undercut distance, in devices such as SOI sensors, the proof mass is lower for the same device area than would be the case if the undercut distance were not limited. Consequently, the limitations on undercut distance require the device to be larger than would otherwise be necessary.
There is thus a need in the art for a method for producing a MEMS structure on a substrate, and particularly on SOI wafers, such that the undercut distance can be increased without adversely affecting interconnects, isolation structures, and other components on the finished device. There is also a need in the art for a method for reducing the size of SOI sensors and other such MEMS devices by increasing the undercut distance. These and other needs are met by the methodologies and devices disclosed herein and hereinafter described.
In one aspect, a method for creating a MEMS device is provided. In accordance with the method, an article, such as an SOI wafer, is provided which comprises a substrate, a sacrificial layer and a semiconductor layer, wherein the sacrificial layer comprises a first material. A MEMS structure is then formed in the semiconductor layer of the article. The MEMS structure has first and second elements which are separated from each other by a trench that exposes a portion of the sacrificial layer, and in which the first element is attached to the substrate by the sacrificial layer. The first element is then released from the substrate by contacting the sacrificial layer with a first etchant, and is subsequently reattached to the substrate with a second material which is preferably diverse from the first material. Next, metal interconnects, isolation structures, and other such device features are defined on the article, after which the first element is re-released from the substrate by contacting the second material with a second etchant. The etchant used to release the first element from the substrate may also be used to etch the second material, in which case the second material is typically chosen so that it is etched at a faster rate by the etchant than the first material. Thus, for example, if the etchant is an aqueous solution of HF, the first material may be silicon oxide and the second material may be PSG. Since the metal interconnects, isolation structures and other such device features are formed after the first element is initially released from the substrate, and since the re-release of the first element can be accomplished with much shorter etch times, the undercut distance can be increased without adversely affecting these device features, and hence the size of the device can be reduced.
In another aspect, a method for creating a MEMS device is provided. The method utilizes an article, such as an SOI wafer, which comprises a substrate, a sacrificial layer and a semiconductor layer, wherein the sacrificial layer is disposed between the substrate and the semiconductor layer. A MEMS structure is then formed in the semiconductor layer, the MEMS structure having first and second elements which are separated from each other by a trench which exposes a portion of the sacrificial layer. The first element is released from the substrate by contacting the sacrificial layer with a liquid etchant. A backfill material is then deposited in the trench such that the backfill material attaches the first element to the substrate. A metal interconnect is then formed on the article, after which the backfill material is etched.
In still another aspect a method for creating a MEMS device is provided. In accordance with the method, an article is provided which comprises a substrate, a sacrificial layer and a semiconductor layer, wherein the sacrificial layer comprises a first material. A MEMS structure is then formed in the semiconductor layer. The MEMS structure has first and second elements which are separated from each other by a trench. The first element is attached to the substrate by the sacrificial layer, and the trench exposes a portion of the sacrificial layer. The portion of the sacrificial layer which attaches the first element to the substrate is then etched, after which the first element is attached to at least one of the substrate and the second element with a second material. The first element is then released from at least one of, and preferably both of, the substrate and the second element by contacting the second material with an etchant.
These and other aspects are described in further detail below.